A peculiar flaring episode of Cygnus X-1

Recent monitoring of Cyg X-1 with the Rossi X-Ray Timing Explorer (RXTE) revealed a period of intense flaring, which started in 2000 October and lasted until 2001 March. The source exhibited some quite unusual behaviors during this period. The soft X-ray flux of the source went up and down 3 times on a timescale of about 1 month, as discovered by the All-Sky Monitor aboard RXTE, before finally returning to the normal level (of the hard state). The observed spectral and temporal X-ray properties of Cyg X-1 are mostly intermediate between the canonical hard and soft states. This is known previously for strong X-ray flares; however, we show that the source did enter a period that resembles, in many ways, a sustained soft state during the last of the three flares. We make detailed comparisons between this flare and the 1996 state transition in terms of the observed X-ray properties, such as the flux-hardness correlation, X-ray spectrum, and power density spectrum. We point out the similarities and differences, and discuss possible implications of the results on our understanding of the phenomena of flares and state transitions associated with Cyg X-1.